SUPPRESSION OF T-INDEPENDENT IGM XENOANTIBODY FORMATION BY LEFLUNOMIDE DURING XENOGRAFTING OF HAMSTER HEARTS IN RATS

Background. It was recently shown that leflunomide (LF) delayed xenoan tibody (XAb) formation and xenograft (Xg) rejection in a hamster-to-ra t heart transplantation model. Our aim in this study was further inves tigation of the mechanism of LF-mediated suppression of XAb formation. Methods, Hamster hearts were heterotopically transplanted to euthymic or nude rats receiving LF and/or cyclosporine (CsA), Second hamster h earts were transplanted at the time of first Xg rejection, Serum from rejecting rats was transferred to naive rats receiving a hamster heart Xg, The isotype of XAbs was examined by fluorescence-activated cell s orting, Tissue deposition of XAbs and complement was determined by imm unofluorescence, XAb formation and its response to LF were also invest igated in severe combined immunodeficient mice reconstituted with puri fied CD5(+) or CD5(-) rat B cells. Results. After xenografting, untrea ted PVG rats developed high titers of anti-hamster IgM XAbs that appea red T-independent (T-I) as they could not be suppressed by CsA and als o occurred in athymic nude rats. A second Xg transplanted in control o r CsA-treated rats rejecting a first Xg was subject to hyperacute reje ction, Hyperacute rejection also occurred in naive rats after adoptive transfer of serum from rejecting rats, Monotherapy with LF resulted i n a suppression of early IgM XAb formation and in a delay of Xg reject ion, which was associated with predominantly IgG anti-hamster XAbs, Th ese XAbs were T-dependent, as they did not occur in nude rats and were suppressed by CsA. CD5(+) B lymphocytes appeared to contribute to T-I IgM XAb formation, as LF reduced the percentage of peripheral blood C D5(+) B lymphocytes and severe combined immunodeficient mice reconstit uted with purified CD5(+) B cells, but not with CD5(-) B cells, produc ed anti-hamster IgM which were suppressed by LF but not CsA, Conclusio ns. In rats, T-I XAb formation is a first step leading to hamster Xg r ejection and is suppressed by LF leading to prolonged Xg survival.